Electron tubes and methods of operation thereof for energy conversion



p 1956 A. EICHENBAUM I ELECTRON TUBES AND METHODS OF OPERATION THEREOF FOR ENERGY CONVERSION Filed NOV. 30, 1961 INVENTOR. fi/i Z. Aryan 140 x M4 United States Patent 3,274,404 ELECTRON TUBES AND METHODS OF OPERA- TION THEREOF FOR ENERGY CONVERSION Arie L. Eichenbaum, Levittown, Pa., assignor to Radio Corporation of America, a corporation of Delaware Filed Nov. 30, 1961, Ser. No. 156,064 12 Claims. (Cl. 310-4) The present invention relates to electron tubes and methods of operation thereof, and particularly to a high efficiency thermionic energy converter tube with a cathode operated at a temperature resulting in a high rate of evaporation of emissive material.

A typical thermionic energy converter tube comprises an envelope containing an electron emitter or cathode and an anode, and means usually external to the envelope for heating the cathode to electron emitting temperature and for cooling the anode. Due to the difference between the work functions of the cathode and anode the electrons emitted by the cathode travel to the anode charging the latter negatively with respect to the cathode, thus converting part of the input heat energy into electrical output energy. It is customary to provide positive ions in the interelectrode space of the tube to neutralize the space charge of the electrons and thereby increase the electron current to the anode. One way to do this is to provide a quantity of alkali metal vapor within the envelope and use a cathode of a material having an electron work function higher than the ionization potential of the vapor, in which case vapor atoms are ionized by contact with the cathode surface. For example, the cathode may be tantalum, with the work function of 4.1 volts, used with cesium, with an ionization potential of 3.9 volts. The voltage output of the converter depends upon the diiference between the work functions of the cathode and anode. Therefore, the anode is cooled to produce a partial coverage of alkali metal vapor thereon, which reduces its work function, in addition to dissipating the heat generating by electron bombardment.

Efficient operation of a thermionic energy converter requires that the cathode be operated at as high a temperature as possible, since the output power to the load increases faster with increasing temperature than does the radiation loss. Efficient converters operate with cathode work function 2 volts and emit electrons efliciently at cathode temperatures in the range 1400" K. T 3500 K., with the highest efficiency being obtained with the highest possible cathode temperature. However, the lifetime of the cathode is shortened at the highest temperatures due to evaporation of the cathode material. Thus, the present practice involves a compromise between high efliciency and long life, in which the cathode is operated at temperatures producing rates of evaporation of the emissive material from the cathode not greater than about .0002 gram/cmWhour. Such temperatures are well below the melting points of the emissive materials.

The object of the present invention is to provide a high efliciency electron tube having long life.

Another object is to provide an electron tube in which the cathode can be operated with high efliciency at high temperatures producing high rates of evaporation of emissive material without unduly limiting the tube life.

A further object is to provide an electron tube in which the functions of the cathode and anode are interchangeable.

A more specific object is to provide a thermionic energy converter tube in which the cathode can be operated at temperatures near the melting point of the cathode material without unduly limiting the life of the tube.

These and other objects are attained, in accordance with the present invention, by providing an electron tube, such "ice as a thermionic energy converter tube, comprising two similar juxtaposed electrodes, at least one of which comprises electron emissive material and thus is adapted to serve as a cathode, while the other electrode serves as an anode, mounted as portions of the tube envelope, and external means for heating either electrode. In the operation of the tube, the electrode initially having the emissive material is heated externally to maintain it at a high temperature producing copious electron emission and a high rate of evaporation of emissive material for a time suflicient to evaporate and deposit part or all of the emissive material onto the other electrode. Thereafter, the tube is operated in reverse, with the other electrode serving as the cathode and the one electrode serving as the anode, by heating the other electrode. Since the emissive material can be transferred back and forth between the two electrodes indefinitely, the rate of evaporation is not a practical limitation on the cathode temperature insofar as long life is concerned. Hence, the cathode ternperature can be made very high, which results in highly efficient operation of the tube. The cathode temperature is limited only by the melting point of the emissive material used. If the cathode is operated near the melting point of the emissive material, the supports for the two alternate cathode electrodes must be made of a material having a higher melting point than the emissive material. Particularly in the case of a thermionic energy converter tube, external means may be provided for cooling the electrode serving as the anode. When the operation of the tube is reversed, the heating and cooling means may be interchanged (for example, in position). When the tube contains a cesium atmosphere, external means are also provided for cooling the portions of the envelope adjacent to the anode electrode to adjust the vapor pressure of the cesium. When the operation is reversed, this cooling means may be shifted to the opposite side of the tube along with the anode cooling means.

In the accompanying drawing:

FIG. 1 is an axial sectional view of a thermionic energy converter tube embodying the present invention; and

FIG. 2 is a transverse sectional view along theline 22 of FIG. 1.

The invention is illustrated in the drawing as embodied in a thermionic energy converter tube 10 having a vacuum tight envelope 12 made up of two similar end portions 14A and 14B separated by a cylinder or ring portion 16. Each of the end portions 14A and 14B is made up of an annular plate portion 18, a reentrant tubular portion 20, and a fiat disc portion 22. As shown in FIG. 2, the disc portions 22 are parallel to each other and positioned relatively close to each other to form a diode of which the two disc portions are the cathode and anode. The inner wall of one of the two disc portions 22 (the lower one as shown in FIG. 1) is provided with a coating or layer24 of electron emissive material.

Since the disc portions 22 form parts of the tube envelope, the portion 22 on which the emissive material is provided can be heated, to heat the emissive material to the desired temperature, as by radiation from an external heat source 25, as indicated by the arrows in FIG. 1. The heat source 25 may, for example, comprise any chemical (fossil) or nuclear fuel capable of producing the desired cathode temperature. The heat source 25 may be in contact with the disc portion 22, instead of spaced therefrom as shown.

The other disc portion 22, which serves initially as the anode, is also a part of the tube envelope, and hence, it can be cooled by suitable means external to (and separate from) the envelope. FIG. 1 shows a cooling means 26 comprising a relatively thick disc 27 having an internal passageway 28 and two connecting inlet and outlet tubes 30 and 32 for circulating a cooling fluid through the disc. The disc 27 is located within the reentrant portion 20 and preferably kept in contact with the anode portion 22 to dissipate heat by conduction therefrom during operation of the tube.

The tube is operated with the cathode emissive material at a very high temperature, which may be near the melting temperature of the material, in order to attain the highest possible efiiciency, until most or all of the emissive material has been evaporated and condensed on the adjacent anode portion 22. Thereafter, the tube is operated in reverse, with the coated portion 22 serving as the new cathode and the uncoated portion 22 serving as the new anode, by interchanging the positions of the heat source 25 and the cooling means 26 as permitted by the symmetrical design of the envelope portions 14A and 143. This cycle of operation can be repeated indefinitely, for an extremely long tube life at very high efliciency.

For the highest temperature and efficiency, the emissive material should be one having a relatively high melting temperature. When the emissive material is operated near its melting temperature, it is necessary to use materials for the electrode portions 22 and the adjacent supporting portions that have melting points higher than the melting point of the emissive material. The portions 18, 20 and 22 should be of good electrical conductivity, in order to provide low impedance connections to the external load of the tube. The portions 22 should have good heat conductivity for efiicient heat transfer therethrough. The tubular supporting portions 20 should be either of a material having poor heat conductivity or be made very thin, so that they can serve as a heat dam to minimize transfer of heat between the portions 18 and 22 by conduction, and thereby maintain the plate portions 18 at relatively low temperatures. Under these conditions, the thermal efficiency of the tube can be kept high, and also lower melting temperature materials can be used for the plate portions 18 and the ring portion 16.

The ring portion 16 serves to isolate the end portions 14A and 14B from each other electrically, so that a potential difference can be generated between the cathode and the anode (the portions 22) during operation of the tube. The ring portion 16 may be an insulator, such as a high alumina or beryllia ceramic, having a fairly high melting temperature. On the other hand, since the internal and load impedances of a thermionic energy converter are very low, of the order of the tenths of an ohm, the ring portion 16 may be made of a metal, alloy or semi-conductor of suflicient density to maintain a vacuum tight enclosure, providing it is thin enough to present a resistance which is high compared to the parallel load impedance.

An evaporation shield is preferably interposed between the two disc portions 22 and the ring portion 16 to prevent reduction of the surface resistance of the ring portion by evaporation of emissive material thereon. This shield may be made up of two similar coaxial tubular members 34 and 36, one (34) of which has an enlarged extension 38 overlapping the other for a distance sufiicient to completely shield the ring portion 16 from the emissive layer 24, as shown in FIG. 1. The shield members 34 and 36 also function as heat shields for the adjacent tubular portions 20, and therefore, should be capable of operation at temperatures near those of the portions 20.

Preferably, a quantity of alkali metal 40 is provided within the tube envelope to supply positive ions within the interelectrode space between the cathode and the anode for neutralizing the space charge, and also to provide a partial coverage of alkali metal vapor on the anode surface for reducing the work function thereof. Since the metal vapor will condense on the coolest portions of the envelope wall, the vapor pressure is controlled by adjusting the temperature of the coolest portion of the envelope. FIG. 1 shows a spiral cooling pipe 42 in contact with the cooler of the two plates 18 for this purpose. When the tube is operated in reverse, with the lower portion 22 as the anode, the pipe 42 is transferred to the lower plate 18, together with the anode cooling means 26-32. In fact, the pipe 42 and the anode cooling means may be parts of a unitary assembly.

For highest operating temperature (and efficiency) emissive layer 24 preferably comprises a material having a high work function and high melting point, examples of which are shown in the following table:

Melting Temp- Material W.F. (volts) erature (K.)

Tungsten 4. 5 3, 650 Tantalum 4. 15 3, 269 Molybdenun1 4. 2 2, 900

The envelope members 14A and 14B may be made of tungsten carbide or tantalum carbide, having melting temperatures of about 4160 K. and 4150 K., respectively, which are substantially higher than any of the melting temperatures in the above table. On the other hand, the portions 20 and 22 may be made of one of these carbides and the plate members 18 may be made of a lower melting temperature conducting material, such as metallic tungsten, tantalum or molybdenum.

When the emitting material is tantalum operated at 3000 K., the electron current density will be about 53.5 A./cm. which will require a cesium vapor pressure of about 5 10 mm. of Hg for complete neutralization of the space charge. To maintain this vapor pressure, the lowest envelope temperature of the converter tube must be about C. (413 K.). The tantalum vapor pressure at the surface of a tantalum cathode at 3000 K. is only about 4X10 mm. of Hg, and is correspondingly lower at the cooler anode surface. Therefore, the tantalum evaporation does not adversely affect the converter operation. At 3000 K., the rate of evaporation of the tantalum is about .021 gram/cmf hour. Thus, an emissive surface comprising .2 gram of tantalum per square centimeter could be operated for about ten hours at this temperature before most of the tantalum was transferred to the other electrode. By alternating the functions of the two electrodes, the overall life of the tube could be extended to many hundreds of hours. If the anode temperature is 940 K., the percent coverage of cesium on the anode will be about 55%, in which case the work function of the cesiated-tantalum anode will be about 1.8 volts. Thus, the output or load voltage of the converter for maximum power output is 4.11.8=2.3 volts. An efficiency of about 36% is theoretically possible for this example. Operation of a tantalum cathode at temperatures closer to the melting point would yield still higher efficiencies. However, the higher tantalum vapor pressures with higher temperatures would affect the total pressure in the converter and thus interfere with the electron flow process. Therefore, the operating temperature of the tantalum material should not be much higher than 3000 K.

Higher operating temperatures can be used with tungsten as the emissive material. A converter tube with a tungsten surface operated at 3300 K. in a cesium atmosphere having a vapor pressure of 7.2 10- mm. of Hg should have an electron current density of 84.4 A./cm. and an efficiency of 36%. At this temperature, the tungsten will have a vapor pressure of 10" mm. of Hg, which is compatible with the vapor pressure of the cesium, and a rate of evaporation of .18 g./cm. /hour, as compared to .021 g./cm. /hour for tantalum at 3000 K.

Instead of using a pure metal having a high Work function for both electron and ion production for the layer 24, one can provide a layer 24 of a dual work function material, such as a barium oxide impregnated tungsten matrix cathode, on each of the electrodes 22; An impregnated tungsten cathode, sometimes called an L cathode, is a sintered mixture of tungsten powder and barium oxide powder. The exposed surface of this mixture comprises low work function barium oxide portions for emission of electrons and high work function tungsten portions for contact ionizing the cesium vapor to produce the positive ions necessary for space charge neutralization. The normal operating temperature of the L cathode is about 1100 C. The highest reported temperature of an L cathode in a thermionic energy converter is 1300 C. (l5 73 K.), at which temperature the barium evaporation rate is about .00015 g./cm. /hour. Such a cathode having .2 g./cm. of barium oxide, as compared to the tantalum sample, would have a life of about 1333 hours. The estimated efiiciency of the converter at 1300 C. is not more than 8%. When operated in accordance with the present invention at a temperature of 1740 K., for example, the efliciency of the tube would be about 19%, with a barium evaporation rate of .0005 g./ cm. hour.

What is claimed is:

1. The method of operating an electron tube comprising two similar juxtaposed electrodes at least one of which comprises thermionic electron emissive material, comprising the steps of heating said one electrode and the emissive material thereof to an operating temperature above that at which said material evaporates substantially while maintaining the other electrode at a lower temperaturethan said evaporation temperature to receive the condensate of said evaporated material thereon, and thereafter heating said other electrode and the emissive material thus condensed thereon to an operating temperature above that at which said material evaporates while maintaining the said one electrode at a lower temperature than said evaporation temperature to receive condensate of said material thereon.

2. The method of operating an electron tube comprising two similar juxtaposed electrodes at least one of which comprises thermionic electron emissive material, comprising the steps of: maintaining said one electrode and said emissive material at an operating temperature at which said material evaporates at a rate of at least .0005 g./cm. /hour for a time suflicient to evaporate and deposit at least part of said material onto the other electrode, and then maintaining said other electrode and the emissive material thereon at a similar operating temperature, to cause said material to evaporate and deposit back onto said one electrode.

3. The method of operating an electron tube comprising two similar juxtaposed electrodes at least one of which comprises thermionic electron emissive material, comprising the steps of maintaining said one electrode and said material at an operating temperature at which said material evaporates at a rate of at least .0005 g./cm. hour, while cooling the other electrode, for a time sufficient to evaporate and deposit at least part of said material onto said other electrode, and then maintaining said other electrode and the emissive material thereon at a similar operating temperature, while cooling said one electrode, to cause said material to evaporate and deposit back onto said one electrode.

4, An electron tube comprising a symmetrical electrode structure including two similar juxtaposed electrodes, at least one of said electrodes comprising thermionic electron emissive material having a normal operating temperature producing substantial electron emission with a rate of evaporation of said material therefrom not greater than .0002 g./cm. /hour, and means for heating either of said electrodes to maintain it at an operating temperature substantially higher than said normal operating temperature, whereby either of said electrodes can be used as the cathode of the tube.

5. An electron tube comprising a symmetrical electrode structure including two similar juxtaposed electrodes, at least one of said electrodes comprising thermionic electron emissive material having a normal operating temperature producing substantial electron emission with a rate of evaporation of said material therefrom not greater than .0002 g./cm. /hour, means for heating either of said electrodes to maintain it at an operating temperature substantially higher than said normal operating temperature, whereby either of said electrodes can be used as the cathode of the tube, and means for cooling either of said electrodes while the other electrode is being heated.

6. The method of operating an electron tube comprising a symmetrical electrode structure including two similar juxtaposed electrodes and supports therefor, at least one of said electrodes having electron emissive material thereon, said electrodes and their supports having melting temperatures higher than the melting temperature of said emissive material; comprising the steps of maintaining said one electrode and the emissive material thereon at an operating temperature near the melting temperature of said emissive material for a time sufficient to evaporate and deposit at least part of said emissive material onto said other electrode, then maintaining said other electrode and the emissive material thereon at a similar operating temperature, to cause said material to evaporate and deposit back onto said one electrode.

7. A thermionic energy converter tube having an envelope including two substantially fiat portions juxtaposed to each other, electron emissive material having a given melting temperature located on the inner surface of one of said portions, said portions of said envelope being made of a material having good electrical and heat conductivity and a melting temperature higher than said given melting temperature, whereby said tube can be operated with the emissive material near its melting temperature, said envelope further including a ring portion having low electrical conductivity separating said flat portions, and shield means interposed between said fiat portions and said ring portion to prevent deposition of said emissive material on said ring portion.

8. A thermionic energy converter tube having an envelope including two substantially flat portions juxtaposed to each other, electron emissive material having a given melting temperature located on the inner surface of one of said portions, said portions of said envelope being made of a material having good electrical and heat conductivity and a melting temperature higher than said given melting temperature, whereby said tube can be operated with the emissive material near its melting temperature, said envelope further including a ring portion having low electrical conductivity separating said fiat portions, and shield means interposed between said flat portions and said ring portion to prevent deposition of said emissive material on said ring portion, said shield means comprising two overlapping shield rings extending from opposite walls of said envelope and surrounding the space'between said flat portions.

9. A thermionic energy converter tube having a envelope including two substantially flat portions juxtaposed to each other, electron emissive material having a given melting temperature located on the inner surface of one of said portions, said portions of said envelope being made of a material having good electrical and heat conductivity and a melting temperature higher than said given melting temperature, whereby said tube can be operated with the emissive material near its melting temperature, said envelope containing a quantity of an alkali metal having an ionization potential lower than the highest work function of the surfaces within the envelope, external means for heating said one portion to heat said emissive material, external means for cooling the other of said portions, and external means for controlling the vapor pressure of said alkali metal.

10. The method of operating a thermionic energy converter tube having an envelope including two substantially flat portions juxtaposed to each other, electron emissive material having a given melting temperature located on the inner surface of one of said portions, said portions of said envelope being made of a material having good electrical and heat conductivity and a melting temperature higher than said given melting temperature, whereby said tube can be operated with the emissive material near its melting temperature, said envelope containing a quantity of an alkali metal having an ionization potential lower than the highest Work function of the surfaces within the envelope, external means for heating said one portion to heat said emissive material, external means for cooling the other of said portions, and external means for controlling the vapor pressure of said alkali metal; comprising the steps of operating the tube with said emissive material at a temperature near its melting point, with said other portion serving as the anode and with a vapor pressure such that the fractional coverage of said vapor on the anode is at least 50% for a time sufiicient to evaporate and deposit at least part of said emissive material on said other portion, and then interchanging said heating and cooling means and operating the tube under similar conditions with said one portion serving as the anode.

11. A thermionic energy converter having an envelope including two substantially fiat portions juxtaposed to each other, thermionic emissive tantalum having a given melting temperature located on the inner surface of one of said portions, said portions of said envelope being made of tantalum carbide having good electrical and heat conductivity and a melting temperature higher than said given melting temperature, whereby said tube can be operated with the emissive tantalum near its melting temperature, said envelope containing a quantity of cesium having an ionization potential lower than the highest work function of the surfaces within said envelope.

12. An electron tube comprising a symmetrical electrode structure including two similar juxtaposed electrodes and supports therefor, at least one of said electrodes having thermionic electron emissive material thereon, said electrodes and their supports having melting temperatures higher than the evaporation temperautre of said emissive material, and means for maintaining said one electrode at a temperature above said evaporation temperature while cooling the other of said electrodes, to evaporate said emissive material onto said other electrode, whereby said other electrode can be used subsequently as the cathode of the tube.

References Cited by the Examiner UNITED STATES PATENTS 1,407,061 2/ 1922 Gray 313-246 1,965,584 7/1931 Foulke 313-213 2,634,383 4/ 1953 Gurewitsch 313246 X 2,881,384 4/1959 Durant 322-2 2,980,819 4/1961 Feaster 313212 3,021,472 2/ 1962 Hernq-uist 310-4 3,054,914 9/ 1962 Hatsopoulos 3104 3,056,912 10/1962 Forrnan 3104 X OTHER REFERENCES Publication: Direct Conversion of Heat to Electricity, by Kaye and Welch (TK 2950 K38), pages 1-1 to 1-14.

MILTON O. HIRSH'FIELD, Primary Examiner.

ORIS L. RADER, DAVID X. SLINEY, Examiners. 

1. THE METHOD OF OPERATING AN ELECTRON TUBE COMPRISING TWO SIMILAR JUXTAPOSED ELECTRODES AT LEAST ONE OF WHICH COMPRISES THERMIONIC ELECTRON EMISSIVE MATERIAL, COMPRISING THE STEPS OF HEATING SAID ONE ELECTRODE AND THE EMISSIVE MATERIAL THEREOF TO AN OPERATING TEMPERATURE ABOVE THAT AT WHICH SAID MATERIAL EVAPORATES SUBSTANTIALLY WHILE MAINTAINING THE OTHER ELECTRODE AT A LOWER TEMPERATURE THAN SAID EVAPORATION TEMPERATURE TO RECEIVE THE CONDENSATE OF SAID EVAPORATED MATERIAL THEREON, 